There's nothing mystical
or complicated about a sextant. All it is, is a device that measures
the angle between two objects.

Background

The sextant allows
celestial objects to be measured relative to the horizon. This
allows for excellent precision. The sextant allows direct
observation of stars which allows it to be used at night. For
solar observations, filters allow observations of the sun.
Since the measurement is relative to the horizon, the measuring
pointer is a beam of light that reaches the horizon. The
measurement is limited only by the angular accuracy of the
instrument. The horizon and celestial object remain steady
when viewed through a sextant, even when the user is on a moving
ship. This occurs because the sextant views the (unmoving) horizon
directly, and views the celestial object through two opposed mirrors
that subtract the motion of the sextant from the reflection.

The scale
of a sextant has a length of one sixth of a full
circle (60°); hence the sextant's name (sextāns,
-antis is the Latin word for "one sixth".
Sir Isaac Newton (1643-1727) invented the principle
of the doubly reflecting navigation instrument (a
reflecting quadrant but never published it.
Two men independently developed the octant around
1730: John Hadley (1682-1744), an English
mathematician, and Thomas Godfrey (1704-1749), a
glazier in Philadelphia.

How to
Use

The sextant makes use of
two mirrors. With this sextant, one of the mirrors (mirror A in the
diagram) is half-silvered, which allows some light to pass through.
In navigating, you look at the horizon through this mirror.

The other
mirror (mirror B in the diagram) is attached to a
movable arm. Light from an object, let's say the sun,
reflects off this mirror.

The
arm can be moved to a position where the sun's
reflection off the mirror also reflects off mirror A and
through the eyepiece. What you see when this happens is
one object (the sun) superimposed on the other (the
horizon). The angle between the two objects is then read
off the scale. What makes a
sextant so useful in navigation is its accuracy.

It can
measure an angle with precision to the nearest ten
seconds.

(A degree is divided into 60 minutes; a minute
is divided into 60 seconds.)

Navigation by Sextant

There's no way around it: Celestial navigation using a
sextant is a complex and involved process that involves
a fair amount of calculating, correcting, referring to
tables, knowledge of the heavens and the Earth, as well
as a lot of common sense. (No
wonder it's been so quickly replaced by the satellite-dependent
Global Positioning System, or
GPS)

But the basic principles behind celestial navigation are
fairly straightforward. Here are a few examples that show how a
sextant can be used to find location...

Finding latitude is easy enough. The first thing you
need to do is measure the angle between the horizon and
the sun when the sun is at its highest point, which is
right around noontime on your watch. A quick look at
your trusty tables tells you which line of latitude the
sun should be above on that particular day. For example,
let's say it's noon on December 21, and the sun is
directly overhead.
Well, on that day the sun is above the Tropic of
Capricorn, so your latitude would have to be 23.5
degrees S.

It's a good thing, if
you're a navigator, that the Earth spins around at such an even
pace. Every hour it moves 15 degrees. This means that if the sun is
above the longitude of 0 degrees at noon, one hour later it will be
above 15 degrees West. Now if you have a chronometer (this is just a
fancy name meaning "extremely accurate clock"), you can find your
longitude. Let's say that the sun is directly overhead and your
chronometer, which was set to noon when you were at 0 degrees, says
it's 3 o'clock position.

This means that three hours ago the sun was overhead at
this latitude at 0 degrees longitude. In those three
hours, the sun moved 15 degrees 3 times, or 45 degrees.
So you're at 45 degrees West. Of course, the fact that
the sun was directly overhead (which very rarely
happens) made it especially convenient for finding your
longitude, but you could have found your longitude
anyway, with the help of your tables.

Celestial navigation is the process whereby angles between objects
in the sky (celestial objects) and the horizon are used to locate
one's position on the globe. At any given instant of time, any
celestial object (e.g. the
Moon,
Jupiter, navigational star
Spica) will be located directly over a particular geographic
position on the Earth. This geographic position is known as the
celestial object’s sub-point, and its location (e.g. its
latitude and
longitude) can be determined by referring to tables in a
nautical or air almanac.

The
measured angle between the celestial object and the horizon is
directly related to the distance between the subpoint and the
observer, and this measurement is used to define a circle on the
surface of the Earth called a celestial line of position (LOP). The
size and location of this circular line of position can be
determined using mathematical or graphical methods (discussed
below). The LOP is significant because the celestial object would be
observed to be at the same angle above the horizon from any point
along its circumference at that instant.

Acknowledgement: This instruction was compiled from data found
in Wikipedia, the free encyclopedia on-line and other sources.
Visit
http://en.wikipedia.org/wiki/Sextant for a more concise
explanation of the sextant and other navigation instruments.

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document for use as an instruction guide and for teaching.